It seems to be totally random which videos work and which ones never start and changing the resolution does not help. The connection is otherwise fine, I can download games off steam fine and iplayer seemingly loads fine (Except at peak times where HD content is buffer-tastic)
Really annoying. I don't own a TV, youtube is my daily source of video content.
Ever since Virgin took over my cable service (I still had an NTL Provided Terrayon modem until a couple of years back!) it's got steadily worse. At this point the 3G connection my smart phone provides is more consistent.
I've been having this issue for the past week too, majority of the time it's fine then suddenly the video simply refuses to load at all, even trying to download the video file directly through a script I have doesn't work.